FIG. 1 depicts wireless terminal 101 and base station 103, which is part of a digital wireless telecommunications system in the prior art. Although base station 103 is stationary, wireless terminal 101 is not, and, therefore, the distance between wireless terminal 101 and base station 103 varies as wireless terminal 101 moves. As the distance between wireless terminal 101 and base station 103 varies, the strength of the signal received by wireless terminal 101 fluctuates as a result of the well-known phenomenon of Rayleigh fading, even though the strength of the signal transmitted by base station 103 is constant.
Base station 103 incorporates two mechanisms for mitigating the effects of Rayleigh fading. The first mechanism is transmit power control, which increases the transmit power when wireless terminal 101 is in a fade and decreases the transmit power when wireless terminal 101 is in the clear. Because the transmit power control reacts to how a previously transmitted signal was received, there is inherently a delay between current circumstances and the remedy for those circumstances.
When wireless terminal 101 moves slowly, and, therefore, enters and exits Rayleigh fades slowly, the transmit power control is typically capable of responding adequately such that the quality of the received signal is maintained. In contrast, as wireless terminal 101 moves more quickly, the transmit power control is less capable of responding adequately and, therefore, the quality of the received signal degrades. FIG. 2 depicts a graph of the quality of the received signal strength due to Rayleigh fading and transmit power control as a function of the speed with which wireless terminal 101 moves with respect to base station 103.
The second mechanism that wireless terminal 101 and base station 103 use for mitigating the effects of Rayleigh fading is data interleaving. As is well-known in the prior art, data interleaving is a technique for the error correction and detection of burst-errors. Data interleaving is efficacious when only a modest portion of the interleaved block is affected by a fade and is generally ineffective when most of the interleaved block is affected by a fade. Therefore, data interleaving is most effective for mitigating the effects of Rayleigh fading when wireless terminal 101 is moving quickly, and is, therefore, entering and exiting Rayleigh fades so quickly that it is unlikely that wireless terminal 101 is in a fade during the transmission of a single interleaved block. In contrast, data interleaving is less effective, or even entirely ineffective when wireless terminal 101 is moving slowly and can be in a deep fade during the entire transmission of a single interleaved block. FIG. 3. depicts a graph of the quality of the received signal strength due to Rayleigh fading and data interleaving as function of the speed with which wireless terminal 101 moves with respect to base station 103.
As can be seen in FIGS. 2 and 3, transmit power control mitigates Rayleigh fading at low speeds and data interleaving mitigates Rayleigh fading at high speeds. For some wireless telecommunications systems, either transmit power control or data interleaving is capable of sufficiently mitigating Rayleigh fading at all speeds. For other systems, however, there exists a gap when neither transmit power control nor data interleaving is capable of sufficiently mitigating Rayleigh fading.
FIG. 4 depicts a graph of the quality of the received signal strength due to Rayleigh fading as mitigated by both transmit power control and data interleaving, as function of the speed with which wireless terminal 101 moves with respect to base station 103. The salient characteristic in the graph of FIG. 4 is the severe dip in signal quality that exists at a narrow range of speeds because neither transmit power control nor data interleaving is capable of sufficiently overcoming the effect of Rayleigh fading. Although the speed at which the dip occurs depends on the specifics of the given system, for IS-95 CDMA systems the dip is most prominent at about 3 MPH. For this reason, the phenomenon is colloquially called the "3 MPH Effect."
It is particularly unfortunate that the dip occurs at 3 MPH for IS-95 CDMA wireless telecommunications systems because that is close to the typical speed (in the direction of the base station) at which cars travel in traffic jams, and people in traffic jams are wont to place wireless calls.
Therefore, the need exists for a technique that mitigates the 3 MPH effect. Clearly, however, the 3 MPH effect can be eliminated in either of three ways. First, the rapidity with which the transmit power control operates can be increased, or, second, the length of the data interleaving block can be increased so that it becomes improbable that wireless terminal 101 could remain in a fade during the transmission of a substantial portion of an interleaved block. Both the transmit power control and the length of the data interleaving block are typically specified by the air interface, which is the specification that the manufacturers of both wireless terminals and wireless infrastructure equipment must design to. Therefore, changing either the transmit power control or the length of the data interleaving block require that the air-interface standard be changed, and, therefore, that billions of dollars worth of wireless telecommunications infrastructure and terminal equipment be upgraded at a potential cost of tens or hundreds of millions of dollars.
The third apparent remedy to the 3 MPH effect is simply to greatly and permanently increase the transmit power of the radiated signal. That remedy, however, has so many interference and health drawbacks associated with it that the remedy is more dangerous than the problem. When the transmit power of the radiated signal is increased, the likelihood increases that the radiated signal will interfere with other signals. Furthermore, an increase in radiated signal strength increases the likelihood that humans will be exposed to dangerous levels of radiation.
Therefore, the need exists for a technique that eliminate the 3 MPH effect without requiring a change to the air interface and without changing the transmit power of the radiated signal.